A dragline bucket is a shovel-like enclosure which is advanced and controlled by flaccid lines, such as chains, cables or ropes. The bucket includes an open end through which earthen material is received and accumulated in the bucket as it is dragged through the ground.
A typical bucket of the prior art is supported in part by a pair of hoist lines which are attached to opposite side walls of the bucket. The hoist lines are, in turn, coupled through a linkage assembly to one or more lift lines which extend down from an overhead boom. A dump line is connected to the front end of the bucket and to a drag line (commonly referred to as a drag rope) used for pulling the bucket through the ground. A medial portion of the dump line is wrapped about the sheave of a dump block which is also connected through a linkage assembly to the lift lines. The tension applied to the dump line by the drag line causes the dump line to raise the front of the bucket. Release of the tension then permits the front of the bucket to tip forward and dump the accumulated load.
In the prior art, the attitude of the dragline bucket varies greatly depending upon the tension in the dump line. The dump line tension varies not only because of changes in the tension of the drag line, but also on account of its orientation relative to the drag line. For example, a greater share of the tension in the drag line is transmitted to the dump line as the forward portion of the dump line (i.e., the portion extending between the drag line and the dump block) approaches an aligned relationship with the drag line, such as in a tightline position near the boom. In this case, the front of the bucket is pulled upward to form a significant rearward cant in the bucket's attitude. However, if the forward portion of the dump line is set at a smaller angle to the drag line, such as in a lower pit position below the distal end of the boom, then less tension is applied to the dump line. As a result, the bucket tips forward and risks spilling part of the accumulated load through the open front end.
U.S. Pat. No. 3,597,865 to Rumfelt discloses a dragline bucket assembly with a device which seeks to maintain the bucket in a proper carrying attitude. The device of Rumfelt comprises a cylindrical member connected to lift lines and a rod movable in the cylindrical member connected to hoist lines. The rod is biased within the cylinder by a spring assembly including a coil spring, an abutment, nuts, and a closure plug. A dump sheave for supporting a dump rope is connected to the exterior of the cylindrical member. In operation, the rod moves outwardly against the spring tension as the bucket is loaded, and thereby increases the distance between the sheave and the bucket. This outward adjustment of the rod increases the moment arm between the bucket and the dump line, and thus reduces the tension needed in the dump line to support the front of the bucket.
However, the use of such a spring is not considered feasible for the loads and stresses involved in a modern dragline operation. For example, loads in the lift lines for large dragline buckets can reach 500,000 pounds or more. Further, the movements associated with the adjustment of the spring in this device are not deemed likely to significantly alter the position of the dump line and produce the desired level of stabilization. Moreover, the device of Rumfelt is complex and susceptible to fatigue and performance degradation over time as the coil spring wears out.
In the prior art, different lengths of dump lines have also been used in an effort to accommodate different working conditions for a dragline bucket. For instance, short lengths of dump line have been used when a bucket is deployed deep in the pit, and long lengths of dump line have been used when the bucket operates close to the boom. However, these arrangements place restraints on the operation of the bucket and increase the bucket's down time to replace the dump lines as needed.